Rapid microfabrication for complex optical surfaces using digital micromirror device

Abstract

This research presents a simple and low-cost method for rapid fabrication of 3D micro-optical elements without employing complex fabrication procedures and conventional photomasks. Unlike a conventional maskless lithography system, we combined gray-scale photolithography using Digital Micromirror Device (DMD) and thermal reflow process to fabricate 3D micro-optical elements with continuous surface profile. The maskless lithography platform is developed using a DMD as a dynamic mask pattern generator, low cost illumination part with UV LED, an optical microscopic imaging system with 40X microscope objective lens, and reflecting mirrors. We employed a low cost white light LED and microscopic imaging system with CCD image sensor to monitor the alignment of dynamic mask pattern on photoresist and the fabricated microstructures at the object plane. All optical components were aligned along the principle ray of the light to focus reflected light from DMD on the thick photoresist surface. The resulting resolution is 1.5 μm. The optimal conditions in UV exposure dose, number of gray-tone levels of dynamic mask, and the temperature and time control during the thermal reflow process are experimentally determined to fabricate 3D micro-optical elements with continuous surface profile. The 3D micro-optical elements such as blazed grating, blazed diffractive Fresnel lens, and microlens are fabri-cated on a thick photoresist-coated silicon or glass substrate to verify the performance of the implemented maskless exposure system. We fabricated complex 3D optical structure with sloped surface profile on 10 μm thick positive photoresist-coated substrate within 20second by using proposed fabrication process with a simple and inexpensive manner. The surface roughness also measured by using atomic force microscopy to verify the performance of the employed thermal reflow process. The resulting average surface roughness of patterned photoresist after thermal reflow process was 2.5nm...